1. Field of the Invention
The present invention relates to a solid state radiation detector having an electric accumulator for accumulating electric charges, in an amount corresponding to either a dosage of irradiated radiation or luminous energy of light emitted by excitation of the radiation, as electric charges for a latent image.
2. Description of the Related Art
Presently, in the field of radiography for the purpose of medical diagnosis or the like, there are proposed various radiation image recording and retrieving apparatuses, each of which uses a solid state radiation detector (hereinafter simply referred to as a detector) provided with an electrostatic recording medium. The electrostatic recording medium accumulates electric charges obtained by detecting radiation in an electric accumulator as electric charges for a latent image, and the electric charges for a latent image thus accumulated are outputted after being converted into electric signals to represent radiation image information. Although various types of solid state radiation detectors for use in such apparatuses have been disclosed, a typical example is a detector adopting an optical readout mode for retrieving the accumulated electric charges by irradiating retrieving light (an electromagnetic wave for retrieval) onto the detector. Such a mode is adopted in terms of an electric-charge readout process for reading the accumulated electric charges outward.
In Japanese Unexamined Patent Publication Nos. 2000-105297, 2000-284056 and 2000-284057, there are disclosed solid state radiation detectors of an optical readout type, which is capable of striking a balance between a high-speed response upon readout and efficient retrieval of signal electric charges. The above-mentioned solid state radiation detectors refer to an electrostatic recording medium formed by stacking a first electrode having transmissivity with respect to light emitted by radiation for recording or by excitation of the radiation (such light will be hereinafter referred to as recording light); a recording photoconductive layer which takes on conductivity by receiving the recording light; a charge transport layer acting substantially as an insulator with respect to electric charges of the same polarity as the electric charges to be charged on the first electrode and acting substantially as a conductor with respect to electric charges of the reverse polarity to the electric charges of the same polarity; a retrieving photoconductive layer which takes on conductivity by receiving irradiation of retrieving light (an electromagnetic wave for retrieval); and a second electrode having transmissivity with respect to the retrieving light in the order described above, in which signal electric charges (electric charges for a latent image) carrying image information are accumulated in an electric accumulator formed on an interface between the recording photoconductive layer and the charge transport layer.
Moreover, especially in the above-mentioned Japanese Unexamined Patent Publication Nos. 2000-284056 and 2000-284057, there are disclosed electrostatic recording mediums, in which the second electrode having transmissivity with respect to the retrieving light is formed as a stripe electrode including multiple photoelectric charge pair generating line electrodes having transmissivity with respect to the retrieving light, and multiple photoelectric charge pair non-generating line electrodes are provided alternately with and mutually parallel to the photoelectric charge pair generating line electrodes in order to output electric signals at a level corresponding to an amount of the electric charges for a latent image being accumulated in the electric accumulator.
By providing the second electrode with a sub stripe electrode composed of the multiple photoelectric charge pair non-generating line electrodes as described above, a new capacitor is formed between the electric accumulator and the sub stripe electrode, whereby transported charges of the reverse polarity from the electric charges for a latent image accumulated in the electric accumulator by the recording light can be also charged on the sub stripe electrode owing to rearrangement of electric charges upon retrieval. In this way, it is possible to reduce an amount of the transported charges to be distributed to a capacitor formed between the stripe electrode and the electric accumulator via the retrieving photoconductive layer, so that it is relatively smaller as compared to a case where the sub stripe electrode is not provided. As a result, it is made possible to enhance retrieving efficiency by means of increasing an amount of signal electric charges extractable out of the detector, and to strike a balance between a high-speed response upon readout and efficient retrieval of the signal electric charges.
Incidentally, various modes have been disclosed for enhancing an S/N ratio of a detected signal in the electrostatic recording medium provided with the above-described sub stripe electrode.
For example, the amount of signal electric charges extractable outward is increased and the retrieval efficiency is thereby enhanced if the amount of the transported charges to be distributed to the capacitor formed between the sub stripe electrode and the electric accumulator via the retrieving photoconductive layer is larger than the amount of the transported charges to be distributed to the capacitor formed between the stripe electrode and the electric accumulator via the retrieving photoconductive layer. Accordingly, by setting a pair of a photoelectric charge pair generating line electrode and a photoelectric charge pair non-generating line electrode is defined as one cycle, there are disclosed various modes for enhancing the retrieving efficiency and thereby enhancing the S/N ratio, such as a mode of optimizing such a cycle or a mode of optimizing a ratio between a width of the photoelectric charge pair generating line electrode and a width of the photoelectric charge pair non-generating line electrode. However, further enhancement in the S/N ratio is still desired.
The present invention has been made in consideration of the foregoing circumstances. It is an object of the present invention to provide a solid state radiation detector including an electrostatic recording medium provided with a sub stripe electrode, which is capable of further enhancing an S/N ratio thereof.
A solid state radiation detector according to the present invention concerns a solid state radiation detector including an electrostatic recording medium formed by serially stacking a first electrode having transmissivity with respect to recording light, a recording photoconductive layer which takes on conductivity by receiving irradiation of the recording light, an electric accumulator for accumulating electric charges in an amount corresponding to luminous energy of the recording light as electric charges for a latent image, a retrieving photoconductive layer which takes on conductivity by receiving irradiation of retrieving light, and a second electrode provided with multiple photoelectric charge pair generating line electrodes having transmissivity with respect to the retrieving light and multiple photoelectric charge pair non-generating line electrodes in a manner that the photoelectric charge pair generating line electrodes and the photoelectric charge pair non-generating line electrodes are disposed alternately. Here, the solid state radiation detector includes means for detecting electric currents on the photoelectric charge pair generating line electrodes to be connected to the photoelectric charge pair generating line electrodes, means for detecting electric currents on the photoelectric charge pair non-generating line electrodes to be connected to the photoelectric charge pair non-generating line electrodes, and signal synthesizing means for synthesizing a signal detected by the means for detecting electric currents on the photoelectric charge pair generating line electrodes and a signal detected by the means for detecting electric currents on the photoelectric charge pair non-generating line electrodes while inverting any one of the foregoing signals.
Here, the foregoing xe2x80x9celectrostatic recording mediumxe2x80x9d includes the first electrode, the recording photoconductive layer, the retrieving photoconductive layer and the second electrode being stacked in accordance with this order. Moreover, the electrostatic recording medium also includes the electric accumulator formed between the recording photoconductive layer and the retrieving photoconductive layer. Here, the electrostatic recording medium may be formed by additionally stacking other layers, minute conductive members (microplates), or the like. Moreover, the solid state radiation detector may be formed in any types as long as the solid state radiation detector can record image information as an electrostatic latent image by means of irradiating the light which carries radiation image information (the light generated by radiation or excitation of the radiation).
As for a method of forming the above-described electric accumulator, some applicable methods include a method of forming an electric accumulator by providing a charge transport layer and forming an electric accumulator on an interface between the charge transport layer and a recording photoconductive layer (see Japanese Unexamined Patent Publications No.2000-105297 and No. 2000-284056), a method of providing a trap layer and forming an electric accumulator within the trap layer or on an interface between the trap layer and a recording photoconductive layer (see specification of U.S. Pat. No. 4,535,468), a method of providing minute conductive members and the like for condensing and thereby accumulating electric charges for a latent image (see Japanese Unexamined Patent Publication No. 2000-284057), and the like.
Moreover, the foregoing xe2x80x9cphotoelectric charge pair generating line electrodes having transmissivity with respect to the retrieving lightxe2x80x9d refer to electrodes which transmit the retrieving light and thereby allows the retrieving light to generate charge pairs on the retrieving photoconductive layer. Moreover, the foregoing xe2x80x9cphotoelectric charge pair non-generating line electrodesxe2x80x9d refer to electrodes for effectuating output of electric signals at a level corresponding to the amount of the electric charges for a latent image accumulated in the electric accumulator. It is preferred that the photoelectric charge pair non-generating line electrodes possess a light-shielding property against the retrieving light. However, the photoelectric charge pair non-generating line electrodes do not always have to possess a light-shielding property if a light-shielding film having a light-shielding property or the like is provided between the photoelectric charge pair non-generating line electrodes and retrieving light irradiating means. Here, the xe2x80x9clight-shielding propertyxe2x80x9d is not only limited to a property that shields against the retrieving light completely to not generate the charge pairs at all, but the light-shielding property may also include slight transmissivity with respect to the retrieving light so far as the charge pairs that are generated by such transmissivity are virtually insignificant. Therefore, the charge pairs to be generated on the retrieving photoconductive layer are not only limited to those attributable to the retrieving light which passes through the photoelectric charge pair generating line electrodes, but the charge pairs may be also generated on the retrieving photoconductive layer by the retrieving light which slightly passes through the photoelectric charge pair non-generating line electrodes.
Furthermore, it is satisfactory if the foregoing xe2x80x9cretrieving lightxe2x80x9d can effectuate charge transfer in the electrostatic recording medium so as to read the electrostatic latent image electrically. To be more precise, light, radiation and the like are usable as the retrieving light.
According to the solid state radiation detector of the present invention, the signal detected by the means for detecting electric currents on the photoelectric charge pair generating line electrodes to be connected to the photoelectric charge pair generating line electrodes and the signal detected by the means for detecting electric currents on the photoelectric charge pair non-generating line electrodes to be connected to the photoelectric charge pair non-generating line electrodes are synthesized by the signal synthesizing means while inverting any one of the foregoing signals. In this event, the signal corresponding to the electric charges for a latent image to be detected by the means for detecting electric currents on the photoelectric charge pair generating line electrodes and the signal corresponding to the electric charges for a latent image to be detected by the means for detecting electric currents on the photoelectric charge pair non-generating line electrodes have substantially the same waveforms with reverse polarities from each other. Meanwhile, common-mode noises to be detected in common by both of the detecting means, the noises which are immixed owing to electromagnetic induction from a detection circuit itself, an external circuit, a power source or the like, have substantially the same waveforms with the same polarity to each other. Accordingly, if anyone of the signals is inverted and then synthesized with the other, the signal corresponding to the electric charges for a latent image is substantially doubled; meanwhile, the common-mode noises are offset and disappear. In the meantime, white noises N1 and N2 generated by an amplifier itself and detected by both of the detecting means are mutually flat within a certain bandwidth in terms of the frequency. Accordingly, if the white noises N1 and N2 of the same degrees are synthesized together (assuming that N1xe2x89xa6N2xe2x89xa6N), the aggregate white noise is magnified by about {square root over (2)} times as shown in the following formula (1).
For this reason, it is possible to enhance the S/N ratio by {square root over (2)} times or more by synthesizing the signal detected by the means for detecting electric currents on the photoelectric charge pair generating line electrodes and the signal detected by the means for detecting electric currents on the photoelectric charge pair non-generating line electrodes while inverting any one of the foregoing signals.
N1+N2={square root over (N12+N22)}={square root over (2)}xc2x7Nxe2x80x83xe2x80x83(1)